Conventionally, as a cap sealing an opening of a bottle filled with a beverage, a cap has been proposed which includes an accommodation portion accommodating a powdered or liquid raw material therein (for example, refer to Patent Document 1). In the cap, the accommodation portion is opened concomitantly with an opening operation, and the raw material is discharged into the bottle, so that the material is mixed with the beverage.
More specifically, an inner cylinder member including the accommodation portion is attachably/detachably and threadedly mounted on an outer cylinder member fixed to the opening of the bottle. Further, the accommodation portion of the inner cylinder member is sealed in such a manner that the lower end thereof is press-inserted and fitted to a bottom cover member.
In this kind of cap, a problem usually arises in air-tightness of the accommodation portion. The invention disclosed in Patent Document 1 is made to improve the air-tightness of an accommodation portion by integrally forming a packing with an outer peripheral upper edge and an inside portion of an annular groove that allows press-inserting a lower end of the accommodation portion, the material of the packing being softer than that of a bottom cover member.